Precise Positioning Service Performance Standard - GPS.gov

February 2007
GPS PPS PS
SECTION 2.0 PPS SIS Characteristics and
Minimum Usage Assumptions
This section provides an overview of the PPS SIS interface characteristics and the assumptions
made to arrive at the performance standards. The representative receiver characteristics are used
to provide a framework for defining the PPS performance standards. They are not intended to
impose any minimum requirements on receiver manufacturers or integrators, although they are
necessary attributes to achieve the PPS performance described in this document. Receiver
characteristics used in this standard are required in order to establish a frame of reference in which
the PPS SIS performance can be described.
2.1 PPS SIS Interface Specification (IS) and Interface Control Document
(ICD) Requirements
The PPS SIS shall comply with the technical requirements related to the interface between the
Space Segment and the PPS receivers as established by IS-GPS-200, ICD-GPS-224, and ICD-
GPS-225. See Section 4 for the specific references to the particular versions of these ISs/ICDs. In
the event of conflict between the PPS SIS interface characteristics described in this document and
the ISs/ICDs, defer to the ISs/ICDs.
2.2 Overview of PPS SIS Interface Characteristics
This section provides an overview of the PPS SIS interface characteristics. PPS SIS interface
characteristics are allocated to two categories: (1) carrier and modulation radio frequency (RF)
characteristics, and (2) the structure, protocols, and contents of the NAV message.
2.2.1 PPS SIS RF Characteristics
Navstar satellites transmit right-hand circularly polarized (RHCP) L-band signals at frequencies
known as L1 at 1575.42 MHz and L2 at 1227.6 MHz as specified in IS-GPS-200. These signals
are transmitted with enough power to ensure the minimum signal power levels of -158.5 dBW for
L1 C/A-code, -161.5 dBW for L1 P(Y)-code, and -164.5 dBW for L2 P(Y)-code under the conditions
defined in IS-GPS-200. The PPS SIS generation and transmission process is represented in
Figure 1.5-1.
L1 consists of two carrier components which are in phase quadrature with each other. Each carrier
component is bi-phase shift key (BPSK) modulated by a separate bit train. One bit train is the
Modulo-2 sum of the P(Y)-code and NAV data, while the other is the Modulo-2 sum of the C/Acode
and the NAV data. The L2 link is BPSK modulated by only one of those two bit trains; the bit
train to be used for L2 modulation is selected by ground command. A third modulation mode is
also selectable on the L2 channel by ground command: it uses the P(Y)-code without the NAV
data as the modulating signal. For a particular Space Vehicle (SV), all transmitted signal elements
(carriers, codes and data) are coherently derived from the same on-board frequency source. See
IS-GPS-200 for the definition of the P-code and the C/A-code. See ICD-GPS-224 or ICD-GPS-225
for the definition of the Y-code.
1 st Edition Page 8